What's the story with all the Photo bucket "bandwidth exceed" messages showing up in threads instead of photos? I presume it's the owner of the pictures needs to upgrade to "Pro" service? But I use the free PB service and I'm not getting that message in place of my photos....

Those messages tell that the montly bandwidth limit of the account has been reached. The limit resets at the beginning of a new month (determined by the time one registered their account).

While it doesn't mean that the user needs to upgrade to Pro, per se, it would stop the messages with it's unlimited bandwidth (though it's still monitored for "excessive" and abusive use).
The monthly limit for free accounts is 10GB so reaching that would probably require a combination of having a lot of pictures posted on numerous places where lots of people view them.